Figure 1 shows the basic
concepts of how capacitors function.
A dielectric material is layered
between two metal electrodes, and
an electrical charge proportional
to the voltage is stored in the
capacitor when a voltage is applied
across the electrodes.

Figure 1

"C" is the capacitance of the capacitor. The capacitance is calculated
using the equation shown below as a function of the surface area of the
electrodes (S), the distance between the electrodes (t), and the dielectric
constant of the dielectric ().

A larger capacitance can
be obtained by either increasing the dielectric
constant, increasing the electrode surface
area (S), or by decreasing the distance
between the electrodes(t).
The dielectric constant of an
aluminum oxide layer averages
between 7 and 8. The most frequent
dielectric constants of dielectrics
used in capacitors are listed
in Table 1.

Table 1

Dielectric
Material

Dielectric
Constant

Dielectric
Material

Dielectric
Constant

Aluminum
oxide thin film

7
- 8

Porcelain
(ceramic)

10
- 120

Mylar

3.2

Polyethylene

2.5

Mica

6
- 8

Tantalum
oxide film

10
- 20

The effective surface area
of aluminum electrolytic capacitors can
be increased by as much as 120 times. By
roughening the surface of the high-purity
aluminum foil, the process makes it possible
to produce capacitances far larger than
those of other types of capacitors.
Please note that capacitors are
typically described in terms
of the primary dielectric material.
A few examples are "aluminum
electrolytic capacitor" or "tantalum
capacitor."

[2] Aluminum Electrolytic Capacitors

The anode in the aluminum
electrolytic capacitor is made from a high-purity
aluminum foil with an aluminum oxide thin
film dielectric on its surface. The capacitor
is structured using an electrolytic paper
containing an electrolytic solution and
an aluminum electrode foil for contacting
the cathode.
The thickness of the anode oxide
thin film is the distance between
the electrodes (t) in Figure
2 in the section on how capacitors
function. The thickness of the
anode oxide thin film in an aluminum
electrolytic capacitor is selected
by the required withstand voltage.
Large amounts of charge can be
stored in a small capacitor because
the value for t can be made extremely
small. This occurs because the
value for the electrode surface
area (S) can be increased by
roughening the surface, and because
the dielectric constant () is large.

Figure 2

[2]-1 Surface Roughing (Etching)

The raw foil for the anode
uses a high-purity aluminum foil (a minimum
purity level of 99.99%) that is normally
50 to 100 µm thick. The cathode foil
material uses an aluminum foil that is
at least 99% pure and about 15 to 60 µm
thick. Because the capacitance is proportional
to the surface area of the electrodes,
the effective surface area is increased
by roughening (etching) the surface of
the aluminum foil before growing the dielectric
film. Generally, this surface roughening
is referred to as "etching."

There are two typical etching
processes. The first option submerges
the aluminum foil in hydrochloric
acid (physical etching). A secondary
option is electrolysis where
the aluminum as the anode is
placed in an aqueous hydrochloric
acid solution (electrochemical
etching). In electrochemical
etching, the etching profile
will vary depending on factors
such as the waveform of the electrical
current, the composition of the
solution, and the temperature.
The etching method can be determined
by the desired capacitor performance.
Generally, it is possible to
achieve etching multipliers (the
ratio between the surface area
of the smooth foil and the effective
surface area of the etched foil)
approximately between 3 and 120.

The foil is then rinsed thoroughly
with water. Any residual chlorine
ions on the foil's surface after
etching can corrode the foil
and damage the capacitor. After
etching, the foil's surface can
be categorized broadly as shown
below by the selected voltage
at which the capacitor functions
properly. See the magnified view
of the surface in Photograph
1.

Foil
Surface
(3500x Magnification)

Cross-section
of Capacitor
(350x Magnification)

Types
of
Etched Foils

Low-voltage
foil

High-voltage
foil

[2]-2 Forming (Anode Oxidation)

The "Forming" process is
defined by creating an electrically insulating
oxide (to provide the withstand voltage)
on the aluminum surface by performing anode
oxidation in the electrolytic solution
used for the growth. The produced chemical
film is used as the anode thin film.

The anode oxidation, as
shown in Figure 3, is produced by applying
a voltage to the submerged foil found in
the electrolytic solution used for growing
the oxide film. Generally, the electrolytic
solution is an aqueous solution such as
ammonium boric acid, ammonium phosphate,
or ammonium adipic for acid.

Figure 3

During
the anode oxidation (DC electrolysis),
AL2O3 is
produced by a reaction between the water
and the aluminum's Al3+ ions.
The thickness of the grown thin film is
nearly proportional to the applied voltage]
with approximately 1.0 to 1.4 nm per volt.
The chemical reactions on the anode side
and the cathode side are as follows.

Figure 4: The Anode Oxidation
Reaction

Photographs
2 and 3 show a magnified view of the
oxide layer produced through anode oxidation.

Photograph 2: Low-voltage
forming foil

Photograph 3: High-voltage
forming foil

Photograph
2: Low-voltage forming foil

Photograph
3: High-voltage forming foil

Photograph
of surface

Photograph
of surface

[2]-3 Electrolyte

Aluminum electrolytic capacitors
are made by layering the electrolytic paper
between the anode and cathode foils, and
then coiling the result. The process of
preparing an electrode facing the etched
anode foil surface is extremely difficult.
Therefore, the opposing electrode is created
by filling the structure with an electrolyte.
Due to this process, the electrolyte essentially
functions as the cathode. The basic functional
requirements for the electrolyte are as
follows:

(1)

Chemically stable when
it comes in contact with materials used
in the anode, cathode, and electrolytic
paper.

(2)

Easily wets the surfaces
of the electrode.

(3)

Electrically conductive.

(4)

Has the chemical ability
to protect the anode oxide thin film and
compensate for any weaknesses therein.

(5)

Low volatility even at
high temperatures.

(6)

Long-term stability and
characteristics that take into consideration
such things as toxicity.

The grown oxide layer, resulting from the
solute and the solvent (electrolyte),
greatly controls the performance of the
aluminum electrolytic capacitor. The
component materials generally used are
as shown in Table 2.